The present invention relates to a heat mode optical recording/reproducing system using two laser beams and, in particular, it relates to such a system which uses a simple focusing system.
A heat mode optical recording system uses a recording medium which stores data through the physical change of the medium in high temperature. There have been known at least three heat mode recording mediums. One of them is a heat meltable plastic, like styrene. In this case, data is stored by producing a pit on a medum by melting the same by a laser beam, and a plastic medium has the advantage that an erasable recording is possible like a conventional magnetic recording. Another heat mode recording system uses a recording medium which is selectively burnt when illuminated by a laser beam, and a small hole is obtained. That small hole corresponds to each stored data. Still another heat mode recording medium changes the reflection coefficient or the optical transmission coefficient, and transparent portion and opaque portion relates to data 0 and 1.
The present invention may be applied to all of above recording mediums. And for the simplicity of the explanation, erasable medium using plastic is mainly explained in the following description.
Plastics having as a main component styrene ore melted at high temperature. When that meltable plastic is used as a memory medium, data is stored in that medium by illuminating the surface of the plastic with a thin laser beam to provide a pit on the surface of the same. Thus, the presence of a pit shows that the data "1" is stored, and the non-existance of a pit shows that the data "0" is stored. That data stored in the plastic is read out by illuminating the surface of the same with a weak laser beam, which reflects irregularly at a pit, and then, the presence of a read-out laser beam can indicate whether the data stored in the plastic is "1" or "0".
The data thus stored is erased by melting the surface selectively by illuminating the same with a thick and strong laser beam, and cancelling a pit. When a pit is cancelled, new data can be stored again at that portion where a pit existed. Therefore, meltable plastic is an erasable optical memory medium, which can store data repetitively, like a conventional magnetic recording medium.
In order to melt the surface of a plastic, a thin and strong laser beam is necessary, and that beam must have enough energy density to melt the surface. Further, a laser beam must be thin enough (for instance, the diameter of a laser beam is 1 .mu.m) for providing thick data density to increase the storage capacity of the memory. When the diameter of a laser beam is 1 .mu.m, the diameter of a pit is also approximately 1 .mu.m.
When data is erased by cancelling a pit, the stronger laser beam is necessary to melt not only a pit itself but also a peripheral portion of a pit, thus, the diameter of a laser beam for erasing data must be three times as thick as that of a pit, and a laser beam with extremely high power density is required.
However, that high power laser apparatus is very expensive.
Further, a laser beam must be focused accurately on the surface of a plastic, and a prior focusing system is not complete.